In a wireless communication system which uses a multicarrier scheme such as Orthogonal Frequency Division Multiple Access (OFDMA) or Single Carrier-Frequency Division Multiple Access (SC-FDMA), radio resources are defined by a time-frequency region in a two-dimensional space, which is a set of consecutive subcarriers. One time-frequency region is defined by a rectangle determined by time and subcarrier coordinates. That is, one time-frequency region can be defined by at least one symbol in the time domain and multiple subcarriers in the frequency domain. Such a time-frequency region can be allocated to an uplink of a specific User Equipment (UE) or a Base Station (BS) can transmit a time-frequency region to a specific UE in downlink. The number of OFDM symbols in the time domain and the number of subcarriers, starting from a position located at an offset from a reference point in the frequency domain, should be given in order to define a time-frequency region in the two-dimensional space.
The Evolved Universal Mobile Telecommunications System (E-UMTS) system, which is currently under discussion, uses 10 ms radio frames, each including 20 subframes. That is, each subframe is 0.5 ms long. Each resource block includes one subframe and 12 subcarriers, each of which occupies a 15 kHz band. One subframe includes multiple OFDM symbols. Some (for example, the first symbol) of the OFDM symbols can be used to transmit L1/L2 control information.
FIG. 1 illustrates an example structure of a physical channel used in the E-UMTS system, wherein each subframe includes an L1/L2 control information transport region (hatched in the figure) and a data transport region (not hatched in the figure).
FIG. 2 illustrates a general method for transmitting data in the E-UMTS. A Hybrid Auto Repeat reQuest (HARQ) technique, which is a data retransmission method, is used to improve throughput to perform smooth communication in the E-UMTS.
As shown in FIG. 2, to transmit data to a UE according to the HARQ technique, a NODE B transmits downlink (DL) scheduling information through a DL L1/L2 control channel (for example, a Physical Downlink Control Channel (PDCCH)). The DL scheduling information may include a UE identifier (UE ID) or a UE group identifier (Group ID), location and duration information of radio resources allocated for downlink data transmission, transmission parameters such as MIMO-related information, payload size, and modulation method, HARQ process information, a redundancy version, and a new data indicator to identify new data.
Basically, the DL scheduling information can be transmitted through a DL L1/L2 control channel even when retransmission is performed and can be changed according to the channel state. For example, the modulation method or payload size can be changed to transmit data at a higher bit rate if the channel state is better than in initial transmission, while it can be changed to transmit data at a lower bit rate if the channel state is worse.
The BS transmits user data to the UE through a channel resource allocated through the DL scheduling information (for example, a Physical Downlink Shared Channel (PDSCH), which is a physical channel) using the transmission parameters included in the DL scheduling information. The UE monitors a PDCCH every Transmission Time Interval (TTI) to check DL scheduling information destined for the UE and then receives user data transmitted from the BS using the DL scheduling information. Using a UE identifier or a group identifier included in the DL scheduling information, the UE can determine whether or not the DL scheduling information is destined for the UE. When group scheduling using a group identifier is employed, the UE checks scheduling information through a PDCCH using a group identifier allocated to a group including the UE and determines whether or not data received through a PDSCH is destined for the UE using a separate UE identifier allocated to the UE and receives the data if it is destined for the UE.